A cellular telecommunications system is based around cells or similar radio coverage areas. Examples of cellular telecommunications systems include standards such as the GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), AMPS (American Mobile Phone System) or DAMPS (Digital AMPS) or the proposed WCDMA (Wideband Code Division Multiple Access) or UMTS (Universal Mobile Telecommunications System) etc. In general, a cell coverage area or a base station coverage area of a cellular telecommunications system can be defined as a certain geographically limited area covered by one or several base transceiver stations (BTS) serving mobile stations (MS) via an air or radio interface and usually connected to a base station subsystem (BSS). Each of the coverage areas can be controlled by an appropriate controller apparatus. For example, in the proposed WCDMA standard each cell is controlled by at least one radio network controller (RNC) and in the GSM standard each cell is controlled by at least one mobile switching center (MSC). The controller is connected further to a gateway or linking apparatus, such as a serving GPRS support node (SGSN) or gateway mobile switching center (GSMC), linking the cell to the other parts of the communication system. Several cells cover a larger area, and form together the coverage area of a cellular telecommunications network.
The mobile station (MS) or user equipment (UE) within one of the cells of the telecommunications system is correspondingly controlled by the controller of the given cell. Even though the MS may be controlled by only one controller at time, it may also be connected simultaneously to several controllers, e.g. when the cells overlap or in so called soft handoff mode, where the MS may be in communication with two base stations, and those base stations may be connected to different controllers. From these one controller can be defined as the serving (main) controller whereas the others act as secondary controllers.
In the context of the location of a mobile station, and thus the user thereof, the use of cells or similar geographically limited radio coverage areas and associated controllers facilitates the cellular telecommunications system to produce at least a rough location information estimate concerning the current location of an individual mobile station. More particularly, the cellular telecommunications system is always aware of the current location of such mobile stations which are communicating with at least one of the base stations of the system and thus registered within at least one of the controllers of the system (i.e. are located within the area of one cell of the system). This information is available even when the mobile station is located within a coverage area of a visited or “foreign” network, as the visited network is capable of transmitting the location of the mobile station back to the home location register, e.g. for the purposes of routing and charging.
This location information could also be used for other purposes than solely for call processing (routing, charging, resource allocation etc.). There are several possible commercial and non-commercial applications which could use this location information would it be readily available. These possible applications include different local advertisement and information distribution schemes (e.g. transmissions of information directed to those mobile users only who are currently within a certain area), area related WWW-pages (such as time tables, local restaurant, shop or hotel guides, maps local advertisements etc.) for the users of mobile data processing devices, location of those who have called to an emergency number and tracking of mobile users by anyone who wishes to receive this information and is legally entitled to obtain it. An application requiring precise and real-time location information of the movement of a mobile station is a mobile station movement prediction feature that could be utilized, for example, in dynamic network resource allocation. There are various other possible uses of the location information and applications which could use the location information, and in general it can be said that all such applications which need a location information concerning the geographical location of the mobile station could find the location information provided by means of a telecommunications system useful. The usability of this location information could even be substantially increased by improving the accuracy of the location information provided by the telecommunications system.
There is a proposal for a location service feature provided by means of a cellular telecommunications network which could provide the last known location of a mobile station together with a time-stamp. This feature can be provided by a separate network element or server which receives the information from the various controllers of the system. For example, in the GSM this information can be obtained from a Visitor Location Register (VLR) of the visited MSC or the Home location Register (HLR) of the home network. This proposal as such would give the accuracy of one base station or cell, i.e. it would indicate that the mobile station is (or at least was) within the coverage area of a certain base station or cell. When the last coverage area within which the mobile station is positioned is known by the system, an appropriate processor facility may then define the rough geographical location of the mobile station on the basis of the radio coverage area information.
The accuracy of the location determination can be improved by utilizing results of measurements which define the travel time (or travel time differences) of the radio signal sent by the mobile station to the base station. The measurements are preferably accomplished by at least three different base stations covering the area in which the mobile station is currently located. The measurement by each of the three base stations gives the distance (range) between the base station and the mobile station or distance difference (range difference) between the mobile station and two base stations. Each of the range measurements generates a circle centered at the measuring base station. Each of the range difference measurement creates a hyperbola (not a circle as in the range measurements). Thus if range differences are used in the location calculation, the intersections of the hyperbolas are searched for. In an ideal case and in the absence of any measurement error, the intersection of the three circles by the three base stations or the hyperbolas would unambiguously determine the location of the mobile station.
However, the direct radio propagation path between the transmitting and receiving stations may be intermittently or, quite possibly, continuously blocked. This non-line of sight (NLOS) phenomenon is known to be a major source of error in position location because it causes the mobile station to appear further away from the base station than it actually is. Thus, even though the distance measurement results derived from several base stations are utilised the location determination may still give an incorrect location. This is especially the case in a dense urban environment in which several obstacles may cause the mobile station to repeatedly and/or continuously lose the direct line of sight with one or several of the base stations. This causes an increased path length the radio signal has to travel between the transmitting station and the receiving station in order to circumvent all these obstructing elements. Due to reflection and diffraction, the first arriving wave may travel excess path lengths on the order of hundreds of meters if the direct path is blocked.